SF: use peak refresh rate for HWC backpressure

The logic we have for selecting which fence to use to detect
backpressure replies on the assumption that continuously presents as
the rate of vsync. This is not true for VRR displays.

Bug: 296635687
Test: adb root && adb shell service call SurfaceFlinger 1045 f 0.9
Change-Id: Iaceb45741fd657b8c551f51b3c76a643df116da8